Fundamentally, the forming area of the papermaking machine has been clothed by fabrics woven from synthetic materials, i.e., man-made fibers. The general structure of these fabrics has taken two basic forms -- the first comprising a monoplane fabric and the second a double layer or duplex fabric. In a monoplane fabric the woven members travel through the fabric passing from one surface to the other surface in each repeat of the pattern across the width and length of the fabric. The successive warp or filling members will lie side-by-side as near to the center plane of the fabric as the balance in the weave pattern will permit, with the warp and filling yarns interlaced. Thus, the weave pattern, float length and stiffness/diameter of the yarn are the controlling factors which establish the parameters of the fabrics which can be formed. In order to achieve greater strength, stiffness and service life, a double layer or duplex fabric has been used.
A duplex fabric is one in which greater stiffness and strength is obtained by using stacked filling yarns, i.e., sets of filling yarns which are stacked one above the other in two planes. In a duplex fabric, the filling yarns do not interlace from surface to surface; rather, the warp yarns form a double house for the filling yarns in such a way that the sets of filling yarns remain directly over and under each other, the warp yarns crisscrossing between the filling yarns on each side of the fabric, the warp yarns thereby locking the filling yarns in their over and under configuration.
A typical duplex fabric, identified as "prior art", is illustrated in FIG. 1. As seen therein, warp yarns 1, 2, 3 and 4 (which in use lie in the cross-machine direction when the fabric is endless and in the machine direction when the fabric is woven flat) pass between the sets of filling yarns, the yarns 6 and 7 in each set being stacked one above the other in spaced apart planes. The weave illustrated produces an identical pattern on each surface of the fabric. A duplex pattern of this character has been found to have certain disadvantages, particularly when used as a forming fabric. One of the disadvantages results from the cross-machine knuckles which are formed at the points 8 where the warp yarns pass around the filling yarns 7 on the bottom surface or machine side of the fabric. These knuckles are particularly subject to wear and offer minimal protection to the load bearing machine direction yarns 7. In addition, the knuckles coincide with and accentuate the straight and rigid machine direction yarns 7 and create tracking and roll oscillation problems.
Another problem inherent in duplex weave patterns currently in use is the presence of open areas or pockets, indicated at 9 in FIG. 1, which in numerous instances create fabrics having an excessively open construction which causes dimensional instability. In addition, where such open areas exist, reactive forces are captured within the cross-machine yarns 1, 2, 3 and 4 as they cross and interlace between the sets of machine direction yarns 6 and 7. These reactive forces create rigidity relative to any two sets of machine direction pairs, and this restrictive condition in a fabric which inherently has little cross-machine stability prevents the flow and redistribution of the stress producing forces, thereby contributing to the formation of undersirable pockets, roping and wrinkles.
The present invention seeks to overcome the foregoing disadvantages by providing biplanar fabrics which close the objectionable open areas and at the same time provide greater fabric life, particularly on the machine wear surface, as well as better tracking and smoother running with less fatigue related problems.